Due to increasing communication traffics in the optical network, improvement of transmission performance (for example, transmission distance and transmission capacity) of the optical transmission system is expected. A wavelength division multiplexing (WDM) transmission technique is known as one of techniques for improving the transmission performance.
In order to further improve the transmission performance, the recent WDM transmission technique has been studied, for example, for improvement of frequency utilization efficiency in the transmission band by narrowing the wavelength arrangement spacing, and achievement of higher transmission bit rate per wavelength (or may be referred to as a “channel”), as well as increase of the number of wavelengths multiplexed.
For example, the digital coherent transmission technique is considered to be a prospective one of the techniques for improving transmission performance of the optical transmission system. In the digital coherent transmission technique, the received light is subjected to coherent detection by the optical receiver, and a signal subjected to digital sampling is demodulated by digital signal processing.
As examples of the prior art, Japanese Laid-open Patent Publication Nos. 2013-162182, 2010-080665, and 2012-050140 are known.